Commonly owned U.S. Pat. No. 3,891,354, the entire disclosure of which is incorporated herein by reference, teaches a regulating arrangement for regulating the pressure and torque of at least two adjustable-stroke pumps driven by a common drive machine. Each adjustable-stroke pump is provided with a regulator of its own. Each regulator is operative for regulating the pump output pressure by automatically adjusting the stroke of the pump. The regulating arrangement of that patent is furthermore provided with means for detecting overload of the common drive machine for the pumps. When the common drive machine is about to become overloaded, the overload detector activates an antioverload arrangement which alters the operation of one or more of the pressure regulators in a sense causing the strokes of the associated pumps to be automatically decreased until such a point as the overload situation has been corrected.
If an arrangement of the type disclosed in that patent comprises two hydraulic motors, each driven by one of two adjustable-stroke pumps, and respectively associated with the left and right caterpillar tracks, or the left and right wheels, of a vehicle, then steering of the vehicle may become extremely difficult in certain situations. For example, if the resistance to travel encountered by the left caterpillar track suddenly increases, without a corresponding increase in the resistance encountered by the right track, then the loading of the left hydraulic motor and left drive pump will suddenly increase relative to the loading of the right motor and right drive pump. The pressure regulator associated with the left drive pump will compensate by automatically decreasing the stroke of the left pump. However, the stroke of the right pump will not be correspondingly decreased. Inasmuch as both pumps are driven at the same rotary speed by a common drive machine, the volumetric output of the left pump will suddenly drop considerably below the volumetric output of the right pump. As a result, the speed of the left hydraulic motor will suddenly drop considerably below the speed of the right hydraulic motor, and the vehicle will abruptly swerve to the left. This is both annoying and potentially dangerous.
In the aforementioned patent, this situation is even worse, because of the provision of the anti-overload arrangement. If a load is suddenly applied to just one of the left and right drive pumps, and if the load is so great as to threaten to overload the common drive machine, then the anti-overload arrangement alters the operation of the left pressure regulator in a sense causing an even faster stroke decrease of the left pump than would occur under ordinary operation of the left pressure regulator. This can cause particularly violent swerving of the vehicle.